Bugfix: Don't read password from stdin for `backup --stdin`

The `backup` command when used with `--stdin` previously tried to read first
the password, then the data to be backed up from standard input. This meant
it would often confuse part of the data for the password.

From now on, it will instead exit with the message `Fatal: cannot read both
password and data from stdin` unless the password is passed in some other
way (such as `--restic-password-file`, `RESTIC_PASSWORD`, etc).

To enter the password interactively a password command has to be used. For
example on Linux, `mysqldump somedatabase | restic backup --stdin
--password-command='sh -c "systemd-ask-password < /dev/tty"'` securely reads
the password from the terminal.

https://github.com/restic/restic/issues/2591
https://github.com/restic/restic/pull/4011
